This invention relates to miniaturized high frequency circuits and more particularly to the interconnection of these circuits.
Referring to FIG. 1, in a multi-substrate radio frequency (RF) structure, an RF signal is routed off of one substrate 100 onto another substrate 200 and then back onto the first substrate 100. For example, in a radio transceiver, the active RF functions, such as amplifying in a generating circuit 102 and mixing in a receiving circuit 103, are fabricated on one semiconductor substrate 100 and a passive band-pass filter 101 having a certain delay is fabricated on the other substrate 200. In this case, the main RF signal passes off of the first substrate 100, is filtered or otherwise simply processed on the second substrate 200, and returns to the first substrate 100 for further processing.
This routing of the RF signal of FIG. 1 and its subsequent selectivity performance, as seen in FIG. 12, will henceforth be defined as "reentrant off-chip RF selectivity". RF selectivity is the characteristic which determines the extent to which the desired signal can be differentiated from the disturbances of other frequencies or out-of-band signals. In other words, selectivity is related to the amount of attenuation of out-of-band signals.
Generally, the active RF portion of a radio front-end of the transceiver is fabricated on a semiconductor substrate that is physically small relative to the off-chip or second substrate. This small side of the active or first substrate requires any electrical connections to be closely spaced on the semiconductor substrate. Henceforth, "closely spaced electrical connections" are defined as electrical connections which have connection areas that are spaced distally on the same order as their side length.
In prior art reentrant off-chip schemes, poor RF selectivity performance has resulted due to the cross-talk coupling or jumping of the RF signal from the input to the output of the filter at the closely spaced electrical connections, in essence by-passing the filter. Crosstalk coupling is the signal loss of the desired signal caused by the interference of stray electromagnetic coupling of energy from one circuit to another, i.e., inductive and capacitive coupling existing between closely adjacent parallel electrical connections such as bonding pads. Viewed from a different perspective, when routing a signal off-chip through a short electrical delay of the filter, the mingling of the signals between the input and output of the signal processor, each contributing to the cross-talk coupling, can cause severe degradation in selectivity or can otherwise degrade the desired delay or timing performance of the off-chip functional block.
The problem of crosstalk with reentrant RF connections, increases as the RF electrical connections are spaced closer. The strongest mode or one cause of cross-talk coupling with reentrant RF connections is known as common mode impedance coupling and often experienced in a single ended circuit.
Single ended circuits generate or receive a single alternating current (AC) signal which is referenced to a common ground. Many, or all, circuits in such a single-ended system share a common ground, and all the signal currents return to their originating circuits through this common ground. However, any shared ground has a non-zero impedance which is common to all circuits, and this common impedance provides a path through which signals can couple add ohmic loss occurs.
In the case of reentrant off-chip RF selectivity of FIGS. 1 and 12, the RF signal generated by the generating circuit 102 will be coupled into the load or the receiving circuit 103 as a function of the ohmic loss or common impedance 104, as represented schematically in FIG. 1 and the, resultant response shown in FIG. 12. Trace 1201 of FIG. 12 represents a typical frequency response of the filter 101 alone having about 80 dB of out-of-band attenuation, and trace 1200 represents a typical degraded response with the common impedance 104 added to the filter 101. Therefore, the maximum out-of-band attenuation or selectivity of the band-pass filter 101 will not be realized with the single-ended circuit of FIG. 1. Instead, the cross coupling generally results in out-of-band attenuation, or selectivity, of only approximately 30 dB as seen in trace 1200 of FIG. 12.
Conventionally, the efforts to increase the selectivity have focused on improving grounding and providing multiple ground straps to reduce the cross coupling. However, these efforts alone have failed to produce any significant improvement in selectivity because single-ended circuits were still used in these reentrant off-chip schemes. Accordingly, it is desired to provide an improved reentrant off-chip RF selectivity structure that reduces crosstalk significantly at the filter connections for RF processing but is not limited to frequency selective filtering.